1. Field of the Invention
The present invention relates to a defect inspection method and apparatus which detects a difference between corresponding signals, compares the detected difference with a threshold value, and judges the part under examination to be defective if the difference is larger than the threshold value. More particularly, the invention relates to an image defect inspection method and apparatus which detects a gray level difference between corresponding portions of two images, compares the detected gray level difference with a threshold value, and judges one or the other of the portions to be defective if the gray level difference is larger than the threshold value, and also relates to an appearance inspection apparatus which, by using such a method, detects a defect in a semiconductor circuit pattern formed on a semiconductor wafer. Still more particularly, the present invention relates to a technique for determining such a threshold value in accordance with the above signals (images).
2. Description of the Related Art
The present invention is directed to an image processing method and apparatus which compares corresponding portions between two images that should be the same, and judges any portion where the difference is large to be defective. The description herein is given by taking as an example an appearance inspection apparatus (inspection machine) for detecting a defect in a semiconductor circuit pattern formed on a semiconductor wafer during a semiconductor manufacturing process, but the invention is not limited to this particular type of apparatus. Generally, a bright field inspection apparatus, in which the surface of a sample is illuminated from a vertical direction and the image of its reflected light is captured, is employed for such an appearance inspection apparatus, but a dark field inspection apparatus which does not directly capture the illumination light is also used. In the case of the dark field inspection apparatus, the surface of the sample is illuminated from an oblique or a vertical direction, and a sensor is disposed so as not to detect specularly reflected light. Then, the dark field image of the surface of the sample is obtained by sequentially scanning the surface with the illumination light. Accordingly, some dark field apparatuses may not use an image sensor, but it will be appreciated the present invention is also applicable to such apparatuses. In this way, the present invention is applicable to any image processing method and apparatus as long as the method and apparatus are designed to compare corresponding portions between two images (signals) that should be the same, and to judge any portion where the difference is large to be defective.
In a semiconductor manufacturing process, many chips (dies) are formed on a semiconductor wafer. Patterns are formed in multiple layers on each die. Each completed die is electrically tested using a prober and a tester, and any defective die is removed from the assembly process. In a semiconductor manufacturing process, the manufacturing yield is a very important factor, and the result of the electrical testing is fed back to the manufacturing process and used for the management of each process step. However, as the semiconductor manufacturing process consists of many process steps, it takes a very long time before the electrical testing can be conducted after the manufacturing is started. Therefore, when, for example, a certain process step is found faulty as a result of the electrical testing, many wafers are already partway through the process and, thus, the result of the electrical testing cannot be easily utilized to improve the yield. In view of this, pattern defect inspection is performed to inspect formed patterns in the middle of the process in order to detect pattern defects. If the pattern defect inspection is performed at a plurality of steps in the manufacturing process, it becomes possible to detect defects that occurred after the preceding inspection, and the result of the inspection can thus be promptly reflected in the process management.
In an appearance inspection apparatus currently in use, a semiconductor wafer is illuminated, an image of a semiconductor circuit pattern is optically captured, and an electrical image signal is generated which is further converted into a multi-valued digital signal (digital gray level signal). Then, a difference signal (gray level difference signal) is generated that represents the difference between the gray level signal of the pattern under inspection and the gray level signal of a reference pattern, and any portion where the difference is larger than a predetermined threshold value is judged to be defective.
Generally, the reference pattern is a neighboring die or a neighboring similar pattern. Then, a defect grouping process is performed in which the portion that has been judged to be defective is examined in further detail to determine whether the defect is a true defect that affects the yield. The defect grouping process takes a long time because each defective portion needs to be examined in detail. Therefore, in the defect judgment, it is required that any true defect be invariably judged to be a defect, while minimizing the possibility of judging any defect other than a true defect to be a defect.
To achieve this, an optimum setting of the threshold value is critical. If the threshold value is set too small, the number of pixels judged to be defective will increase, and portions not truly defective will be judged defective, thus resulting in the problem that the time required for the defect grouping process increases. Conversely, if the threshold value is set too large, even true defects may be judged not to be defects, resulting in the problem that the inspection is inadequate.
In a prior art method that automatically determines the threshold value according to each sample, the digital gray level signal of the pattern of a similar sample is generated in advance, followed by the generation of a gray level difference signal, and a histogram of gray level differences is created. Then, a variation reference difference, which is set by a prescribed proportion of a portion where the gray level difference is large in the histogram, is obtained, and the threshold value for detection is determined by adding a fixed difference to the reference difference. The reason for this is to prevent the number of pixels judged to be defective from increasing appreciably in cases where the variance of the distribution of the differences is large, by considering that such cases can become a problem in practice. In this method, the variation reference difference varies from sample to sample, but the fixed difference to be added is fixed and does not vary from sample to sample; accordingly, this method has the problem that the proper threshold value cannot be determined when the noise level varies.
To solve the above problem, various methods for determining the threshold value have been proposed. For example, Japanese Unexamined Patent Publication No. H04-107946 discloses a method that determines the threshold value based on the statistics of gray level differences computed at a plurality of portions of a pattern. More specifically, a histogram of maximum values is created by obtaining the maximum value of the gray level difference for each portion. Then, based on the mean and the standard deviation of the gray level difference, the initial value of the optimum threshold value is set, and the optimum threshold value is determined by correcting the initial value based on the number of pixels detected as defective. This method, however, has the following problems: (1) samples must be measured in advance and (2) inspection must be performed a plurality of times. Furthermore, while it is stated that the threshold value at which the number of detected defects suddenly changes is optimum, no description is provided of a specific method for obtaining such a threshold value.
On the other hand, Japanese Patent No. 2996263 discloses a method in which an approximate curve is obtained from the relationship between the gray level difference and its frequency and the gray level difference at which the approximate curve becomes zero is taken as the optimum threshold value. Here, the relationship between the gray level difference and the frequency is represented by a curve, but a curve does not necessarily become zero; therefore, there are cases where the approximate curve does not become zero. Further, even in the case of a straight line, the straight line may not become zero, depending on its slope. Therefore, there can occur cases where the threshold value cannot be set. Furthermore, it is stated that the above curve can be obtained easily, but in actuality, this curve cannot be obtained easily because it depends on the distribution of gray level differences, and hence there arises the problem that the processing time increases.
Japanese Unexamined Patent Publication No. 2002-22421 discloses a method that performs a conversion to an error probability value by using a standard deviation. This method, however, involves the following problems: (1) as the standard deviation is computed directly from the gray level differences, a large amount of computation is required and the processing time increases, and (2) as the error probability value, not the gray level difference, is used to judge the presence or absence of a defect, the error probability value must be computed for every gray level difference, and this again increases the processing time. There is the further problem that, because of the use of the standard deviation, the method is only applicable to normal distributions and cannot be applied to other types of distribution.
For the inspection of semiconductor patterns, etc., it is desired to automate the inspection process, and it is also desired to automatically set the threshold value. To achieve this, there is a need to set the optimum threshold value by instantaneously processing the detected gray level differences and to judge the presence or absence of a defect based on the threshold value; one possible solution here would be to automatically set the threshold value by automatically performing a method such as described above. On the other hand, there is also a need to shorten the inspection time in order to improve throughput, but the above-described methods have problems such as the need to measure the samples a plurality of times in advance, the long processing time, etc. and, therefore, they are not suitable for automating the threshold value setting process in a high-throughout inspection apparatus.
In particular, in the inspection of an actual semiconductor pattern, the noise level differs depending not only on the portion within a die but also on the position of the die on the wafer; furthermore, even when the same semiconductor pattern is formed, the noise level differs from one wafer to another. Accordingly, there is a need to set the optimum threshold value by processing the gray level difference as it is detected, but none of the above-described prior art methods can satisfy such a need.
In view of the above background, the applicant of this patent application proposed the following image defect inspection method in Japanese Unexamined Patent Publication No. 2004-177397. That is, the distribution (histogram) of the gray level difference between corresponding portions of two images is created (see FIG. 1A), and its cumulative frequency is computed (see FIG. 1B). Then, assuming that the gray level difference has a distribution that obeys a prescribed type of distribution, a converted cumulative frequency is computed by converting the cumulative frequency so that the cumulative frequency shows a linear relationship to the gray level difference (see FIG. 1C). After that, an approximate straight line is computed by approximating the converted cumulative frequency by a straight line and, based on the computed approximate straight line, the threshold value is determined from a prescribed cumulative frequency value in accordance with a prescribed calculation method.
For example, in the example of FIG. 1C, the threshold value T is calculated as T=(P1−b+VOP)/a+HO, where “a” is the slope of the approximate straight line, “b” is the intercept at which the approximate straight line intersects the vertical axis, P1 is the cumulative frequency corresponding to the prescribed cumulative probability (p), and VOP and HO are prescribed sensitivity setting parameters.
As the converted cumulative frequency computed with this method shows a linear relationship to the gray level difference, subsequent processing for determining the threshold value is facilitated. As a result, the threshold value can be set automatically in a short time.